Clairmont In-Camera Alexa Filters

Here’s a sneak peek at an article from the September IBC issue of FDTimes on Clairmont’s Behind-the-Lens Filters for Alexa. Our friends at Clairmont Camera never saw a camera they didn’t want to improve. Is it Denny’s California candy-colored streamlined need for speed from those first days at Fiasco Automotive, or an irresistible impulse to see how close they can come to voiding the warranty? No matter. Retrofit syndrome runs rampant at Clairmont Camera. ARRI Alexa is the latest camera to undergo their expert ministrations.

Clairmont Camera’s custom behind-the-lens filter holder for ARRI Alexa. The round dots are magnets that hold the filter in place.

Denny explains, “One of the things cinematographers appreciate most about the ARRI Alexa is the excellent light sensitivity (1600 ISO and beyond). Working at higher ISO ratings can lead to lots of ND if you want to shoot wide open.”

Working closely with Tiffen and Schneider, Clairmont Camera’s Andree Martin and Alan Albert designed custom assemblies that  hold interchangeable filters in front of Alexa’s CMOS sensor assembly (and behind the lens). The filters are optically correct, flat, and manufactured to Clairmont’s exact specifications.

A Clairmont Alexa with filter holder assembly in the lens cavity—in front of the CMOS sensor.

The assemblies enable secure mounting and precise positioning of these custom filters behind the lens. These filter kits are an available option for all of Clairmont’s ARRI Alexa cameras. Modifying an Alexa to accept these behind the lens filter kits is not difficult but needs to be installed by Clairmont’s technical staff.

Currently the following filters are available:

  • Optical Flat
  • IRND 3, 6, 9
  • IRND 3, 6, 9 with Black Promist 1/8
  • Black Promist 1/8
  • IRND 3, 6, 9 with Black Frost 1/8
  • Black Frost 1/8

The custom filter and its filter ring is inserted with an Arricam groundglass tool.

Each filter is permanently mounted in its own frame. Once the camera has been fitted with the internal mounting assembly, filters can be installed or changed in the Alexa by the camera assistant in a few seconds using the Arricam Ground Glass tool included with the filter kit. The mounting assembly can remain in the Alexa all the time, even if the behind the lens filters are not being used.

It is critical to remember that the Flange Depth of the Alexa PL mount must be adjusted to compensate for back focus change caused by the additional glass in the optical path if the behind-the-lens filters are to be used. (The formula is ? the thickness of the glass.) This adjustment is a shop adjustment and not something that can be done by the camera crew in the field.  In order to keep the Back Focus consistent when no other behind-the-lens filter is in place in front of the sensor, an Optical Flat with the same refractive index as the filters is included with each kit.

If this behind-the-lens filter kit is not desired by the DP for a project, Clairmont’s Alexas will simply be rented with the Flange Depth set normally as if the filter retaining assembly was not installed (even though it is) in the camera and no filters will be provided.

Inserting a filter into the Alexa lens cavity.

Note: filters cannot be stacked. That is why combination filters are available: e.g. IRND+Promist, etc. This system reduces the number of filters in the matte-box, possibly reducing some potential vignetting problems, as well as one or two fewer filters that the AC has to deal with when going from one size mattebox to another.

Rene Ohashi, CSC, ASC, David Moxness, CSC, and Jan Madlener, General Manager Clairmont Toronto provided valuable input during the development of this project. Arturo Jacoby took the pictures shown here. Alan Albert and Mardrie Mullen worked on the text. For more information, contact any of Clairmont Camera’s offices.  www.clairmont.com

 

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